Among air-conditioning apparatus, for example, multi-air-conditioning apparatus for buildings, there is a type in which a compressor serving as a heat source is included in an outdoor unit, which is installed outside a construction. When such air-conditioning apparatus performs a heating operation, refrigerant circulating through a refrigerant circuit of the air-conditioning apparatus removes heat from outside air in a heat exchanger of the outdoor unit, and transfers heat to air that is supplied to a heat exchanger of an indoor unit to heat air to be sent into a space to be air-conditioned. Meanwhile, when the air-conditioning apparatus performs a cooling operation, the refrigerant circulating through the refrigerant circuit removes heat from air that is supplied to the heat exchanger of the indoor unit to cool air to be sent into the space to be air-conditioned, and transfers heat in the heat exchanger of the outdoor unit.
When the heating operation is performed with the outdoor unit being installed outdoors, moisture in the air condenses through the heat removal in the outdoor unit and adheres to the heat exchanger of the outdoor unit. When an outside air temperature is low as in winter, the adhering moisture is solidified to form frost. When a large amount of frost adheres to a surface of the heat exchanger, a reduction in heat exchange capacity, a failure of the heat exchanger, and other problems are caused. To address those problems, a defrosting operation is periodically performed to melt and hence remove frost.
In Patent Literature 1, there is disclosed a technology in which, when the defrosting operation is performed, a ventilation function of an air-conditioning apparatus is stopped. Moreover, in Patent Literature 2, there is disclosed a technology in which an absolute humidity is calculated based on a relationship between a temperature around a cooling device and a relative humidity, and it is determined whether or not to start the defrosting operation based on the absolute humidity. In both of Patent Literature 1 and Patent Literature 2, there is performed the defrosting operation in which high-temperature gas refrigerant that has flowed out of the compressor, which has been supplied to the heat exchanger of the indoor unit, is changed in flow direction to flow to the heat exchanger of the outdoor unit, thereby increasing a temperature around a pipe to melt frost.